The software uses external input to construct a pathname that should be within a restricted directory, but it does not properly neutralize sequences such as ".." that can resolve to a location that is outside of that directory.
Extended Description
This allows attackers to traverse the file system to access files or directories that are outside of the restricted directory.
Time of Introduction
Implementation
Applicable Platforms
Languages
All
Common Consequences
Scope
Effect
Integrity
Confidentiality
Availability
Technical Impact: Execute unauthorized code or
commands
The attacker may be able to create or overwrite critical files that
are used to execute code, such as programs or libraries.
Integrity
Technical Impact: Modify files or
directories
The attacker may be able to overwrite or create critical files, such
as programs, libraries, or important data. If the targeted file is used
for a security mechanism, then the attacker may be able to bypass that
mechanism. For example, appending a new account at the end of a password
file may allow an attacker to bypass authentication.
Confidentiality
Technical Impact: Read files or
directories
The attacker may be able read the contents of unexpected files and
expose sensitive data. If the targeted file is used for a security
mechanism, then the attacker may be able to bypass that mechanism. For
example, by reading a password file, the attacker could conduct brute
force password guessing attacks in order to break into an account on the
system.
Availability
Technical Impact: DoS: crash / exit /
restart
The attacker may be able to overwrite, delete, or corrupt unexpected
critical files such as programs, libraries, or important data. This may
prevent the software from working at all and in the case of a protection
mechanisms such as authentication, it has the potential to lockout every
user of the software.
The following code could be for a social networking application in
which each user's profile information is stored in a separate file. All
files are stored in a single directory.
While the programmer intends to access files such as
"/users/cwe/profiles/alice" or "/users/cwe/profiles/bob", there is no
verification of the incoming user parameter. An attacker could provide a
string such as:
(Attack)
../../../etc/passwd
The program would generate a profile pathname like this:
(Result)
/users/cwe/profiles/../../../etc/passwd
When the file is opened, the operating system resolves the "../"
during path canonicalization and actually accesses this file:
(Result)
/etc/passwd
As a result, the attacker could read the entire text of the password
file.
Notice how this code also contains an error message information leak (CWE-209) if the user parameter does not produce a file that exists: the full pathname is provided. Because of the lack of output encoding of the file that is retrieved, there might also be a cross-site scripting problem (CWE-79) if profile contains any HTML, but other code would need to be examined.
Example 3
The following code demonstrates the unrestricted upload of a file
with a Java servlet and a path traversal vulnerability. The action attribute
of an HTML form is sending the upload file request to the Java
servlet.
When submitted the Java servlet's doPost method will receive the
request, extract the name of the file from the Http request header, read
the file contents from the request and output the file to the local
upload directory.
(Bad Code)
Example
Language: Java
public class FileUploadServlet extends HttpServlet {
BufferedWriter bw = new BufferedWriter(new
FileWriter(uploadLocation+filename, true));
for (String line; (line=br.readLine())!=null; )
{
if (line.indexOf(boundary) == -1) {
bw.write(line);
bw.newLine();
bw.flush();
}
} //end of for loop
bw.close();
} catch (IOException ex) {...}
// output successful upload response HTML page
}
// output unsuccessful upload response HTML page
else
{...}
}
...
}
As with the previous example this code does not perform a check on the
type of the file being uploaded. This could allow an attacker to upload
any executable file or other file with malicious code.
Additionally, the creation of the BufferedWriter object is subject to relative path traversal (CWE-22, CWE-23). Depending on the executing environment, the attacker may be able to specify arbitrary files to write to, leading to a wide variety of consequences, from code execution, XSS (CWE-79), or system crash.
Server allows remote attackers to cause a denial
of service via certain HTTP GET requests containing a %2e%2e (encoded
dot-dot), several "/../" sequences, or several "../" in a URI.
Directory traversal vulnerability in FTP server
allows remote authenticated attackers to list arbitrary directories via a
"\.." sequence in an LS command.
The administration function in Access Control
Server allows remote attackers to read HTML, Java class, and image files
outside the web root via a "..\.." sequence in the URL to port 2002.
chain: ".../...//" bypasses protection mechanism using regexp's that remove "../" resulting in collapse into an unsafe value "../" (CWE-182) and resultant path traversal.
Mail server allows remote attackers to create
arbitrary directories via a ".." or rename arbitrary files via a "....//" in
user supplied parameters.
Potential Mitigations
Phase: Implementation
Strategy: Input Validation
Assume all input is malicious. Use an "accept known good" input
validation strategy, i.e., use a whitelist of acceptable inputs that
strictly conform to specifications. Reject any input that does not
strictly conform to specifications, or transform it into something that
does.
When performing input validation, consider all potentially relevant
properties, including length, type of input, the full range of
acceptable values, missing or extra inputs, syntax, consistency across
related fields, and conformance to business rules. As an example of
business rule logic, "boat" may be syntactically valid because it only
contains alphanumeric characters, but it is not valid if the input is
only expected to contain colors such as "red" or "blue."
Do not rely exclusively on looking for malicious or malformed inputs
(i.e., do not rely on a blacklist). A blacklist is likely to miss at
least one undesirable input, especially if the code's environment
changes. This can give attackers enough room to bypass the intended
validation. However, blacklists can be useful for detecting potential
attacks or determining which inputs are so malformed that they should be
rejected outright.
When validating filenames, use stringent whitelists that limit the character set to be used. If feasible, only allow a single "." character in the filename to avoid weaknesses such as CWE-23, and exclude directory separators such as "/" to avoid CWE-36. Use a whitelist of allowable file extensions, which will help to avoid CWE-434.
Do not rely exclusively on a filtering mechanism that removes potentially dangerous characters. This is equivalent to a blacklist, which may be incomplete (CWE-184). For example, filtering "/" is insufficient protection if the filesystem also supports the use of "\" as a directory separator. Another possible error could occur when the filtering is applied in a way that still produces dangerous data (CWE-182). For example, if "../" sequences are removed from the ".../...//" string in a sequential fashion, two instances of "../" would be removed from the original string, but the remaining characters would still form the "../" string.
Phase: Implementation
Strategy: Input Validation
Inputs should be decoded and canonicalized to the application's current internal representation before being validated (CWE-180). Make sure that the application does not decode the same input twice (CWE-174). Such errors could be used to bypass whitelist validation schemes by introducing dangerous inputs after they have been checked.
Use a built-in path canonicalization function (such as realpath() in C) that produces the canonical version of the pathname, which effectively removes ".." sequences and symbolic links (CWE-23, CWE-59). This includes:
[REF-7] Mark Dowd, John McDonald
and Justin Schuh. "The Art of Software Security Assessment". Chapter 9, "Filenames and Paths", Page
503.. 1st Edition. Addison Wesley. 2006.
Description
